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Spellcasting is using the [Z] command to wield innate magical abilities during gameplay. More rarely a player can use the #monster, ^T and #turncommands to unleash quasi-magics.

The first prerequisite for spellcasting is knowing at least one spell. Healers, Monks, Priests, and Wizards are roles which begin the game with knowledge of one or more spells; other classes must find at least one spellbook and read it first.

Pressing [Z] without knowing any spells will produce a message to this effect. Otherwise, a menu will appear of the spells you know, accompanied by their school, level, and chance of failure.

Unless you cancel, the game will first check if you have enough power (magical energy) for the spell selected. If you have enough power, then the game will roll for success or failure. If you succeed, your spell will take effect - see spellbook for an index. If you fail, you will receive the message "You failed to cast the spell correctly"

Spellcasting will burn nutrition with the exception of detect food, and each spell successfully cast will produce skill points equal to its level.

Contents

Spell knowledge only lasts for 20,000 turns after reading a spellbook. Once this time is up, the spell is forgotten, and is marked with an asterisk * in the casting menu. (This is not to be confused with completely forgetting the spell due to amnesia from reading a scroll or being attacked by a mind flayer, which simply erases the spell from your spellcasting menu.) Spellbooks may be re-read once the timeout is at or below 1,000 turns. This will reset the timeout to 20,000 turns[1]. Attempting to read a spellbook before the timeout is low enough will produce the message "You know <spell> quite well already". This will not use up a reading of the spellbook, and if read while owned by a shopkeeper, will not result in a usage fee[2]. Casting a spell when its timeout is at or below 1,000 turns results in the message "Your knowledge of this spell is growing faint.", but the spell can still be cast as normal.

Attempting to cast a forgotten spell will not check any of the usual casting prerequisites, and will consume neither nutrition nor power. It will instead make you confused, stunned, or both. The duration of confusion and stunning is dependent on the level of the forgotten spell, but the choice of effects is not.

Casting a spell costs an amount of power equal to 5 times the spell's level. If you are carrying the Amulet of Yendor, there is an additional penalty of between 1 and 2n power points, where n is the usual power cost.
If you do not have enough power, you will not cast the spell, and you will not use up any power or nutrition in your attempt. A failed cast will only use half the amount of power, rounded down.

Casting any spell other than detect food also costs nutrition. The nutrition cost is twice the power cost (including the Amulet penalty), which means that it is usually 10 times the spell's level. If you are a Wizard with sufficiently high Intelligence, you might be able to take advantage of hungerless or reduced-hunger casting. A failed cast costs just as much nutrition as a successful one.

For this calculation, unskilled maps to 0, basic to 1, skilled to 2, and expert to 3.

If difficulty is positive, the spell is considered difficult. Your success chance is decreased by , with the penalty rounded down.
A difficulty of 1 therefore gives a 53% penalty, and a difficulty of 9 gives a 100% penalty.

If difficulty is zero or negative, the spell is considered easy. Your success chance is increased by , but this bonus will not exceed 20.
A level 1 spell therefore achieves the full 20% bonus with a difficulty of -2. A level 7 spell cannot achieve a better difficulty than -5, which gives a bonus of 10%.

The resultant value is then clamped to lie between 0% and 120% inclusive.

Spells are much easier to cast when their difficulty is zero or less. The following table lists the most difficult spell level that is still considered easy (no penalty for spell level) given particular experience and skill levels.

Highest easy spell level

Xlvl

Unskilled

Basic

Skilled

Expert

1–2

1

2

4

5

3–8

1

3

4

6

9–14

2

3

5

6

15–20

2

4

5

7

21–26

3

4

6

7

27–30

3

5

6

7

If your Intelligence or Wisdom is high, spells of this level or below are likely to have a near-100% base success rate. It is still possible to cast non-easy spells, but they tend to have much lower success rates.

If you have raised your experience level, skill, and spellcasting stat high enough to reach a 100% or higher base success rate, your role and equipment will be the limiting factor in achieving low failure rates.

With a base success rate of 120%, your spellcasting penalty must be 6 or less to achieve an overall 100% success rate. Roles that can achieve this without a robe are Archeologist, Healer, Priest, Tourist, and Wizard. Roles that can achieve this when casting emergency spells or their special spell are Knight and Monk. Roles that can achieve this with their special spell only are Ranger, Samurai, Rogue, and Valkyrie. The remaining two roles, Barbarian and Caveman, cannot achieve 100% success without a robe.

SLASH'EM makes several changes to NetHack's spellcasting system. New spells have been added, and several existing ones have been reshuffled. SLASH'EM roles tend to be more proficient in spell skills than their vanilla counterparts.